With the depleting of traditional fossil fuels and increasing concern on environmental protection, demands for new green efficient energy becomes more and more urgent. The lithium-ion batteries, as a new energy source with a strong competitive advantage, draw extra attention. In addition to the widely application in the prior small and portable batteries, the development of lithium-ion batteries in high power and high energy power batteries is even more desirable.
Power lithium-ion battery requires high current and high-power charge and discharge during use; therefore, new requirements are raised on the electrochemical performance, security and thermal performance of the battery. According to the positive materials, mature power lithium batteries on the market include lithium iron phosphate battery, lithium manganate battery, etc. Conventional methods for improving the performance of lithium power batteries include, such as, the use of new electrode active material, electrolyte, diaphragm, and the use of a wound battery core design or a laminated battery core design, which however provide very limited improvement on the performance of the batteries. The electrode active material is the carrier of the electrochemical reaction in a battery charge and discharge process, and electrode active materials such as lithium iron phosphate, lithium manganate, and lithium titanate are poor in electrical conductivity; therefore, the ideal way to fundamentally improve the performance of lithium power batteries is by use of appropriate methods to improve the electrical conductivity and thermal conductivity of the electrode plate.
In order to improve the performance of lithium power batteries, researchers have considered the following aspects. The first aspect is to improve the conductive property of the electrode active material, such as to improve the conductive property of lithium iron phosphate (e.g., Chinese Patent Publication Nos. CN200710008713.2, CN200410101618.3 and CN200710075736.5), lithium manganate (e.g., Chinese Patent Application Nos. 200410062610.0 and 200710039149.0), lithium titanate, etc. by ion doping, or conductive material coating. However, such a method by improving the conductive property of the electrode active material has become a mature approach which is widely used in production. Accordingly, there is great difficulty in achieving a further breakthrough. The second aspect is to improve the battery production technology by appropriate methods, such as the development of laminated packaging technology (Chinese Patent Publication No. CN1540792A), the use of high conductive electrolyte, high-performance diaphragm (Chinese Patent Application Nos. CN 01130754.4, CN 03126299.6, CN200310117507.7, CN200410078320.5, CN200410057393.6, and CN200410081128.1), and strengthened control over the battery production process. Such a method has achieved some success. The third aspect is to improve the production and processing of the electrode plate, such as to produce an electrode plate with a multi-layer structure, such as lithium manganate layer/lithium iron phosphate layer, and make full use of the safety of the lithium iron phosphate material and high electrical conductivity of lithium cobaltate material, but no significant effect is achieved.
Modification of the electrode plate with high conductivity material may remarkably improve the conductivity of an electrode plate. The graphene material is a lonsdaleite material having a single or a few layers of sp2 hybridized structure, wherein in the multilayer structure, the layers are bound in the form of a π bond therebetween. Since such electrons in the vicinity of Fermi energy are the Dirac electrons having an effective mass of 0, the graphene material has a conductivity of up to 106 S/cm, which is the material with highest conductivity ever discovered. In the Chinese Patent Application Nos. 200910155316.7 and 201010226062.6, LIU Zhao-ping, et al, of Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Science propose a method of improving conductivity of graphene by forming a composite comprising graphene and a polyanion material.